Printed circuit boards (PCB's), including rigid, flexible, single, double and multilayer boards have found important uses in the semi-conductor and electronic industry. In particular, PCB's with cavities are required to housing electronic devices, and to facilitate wire bonding of chips.
Conventional methods of fabricating a PCB board with a cavity involves cutting out a window in an epoxy or dielectric core layer, and another window from an adhesive or bond film. The two windows are then matched or registered before the bond film and the core layer are laminated onto a second core layer. The bond film is positioned in between the two core layers and cures during the laminating process of heating under pressure such that it acts as a fully cured epoxy-glass layer after lamination.
Two types of adhesive or bond films are available on the market. The traditional or "high flow" film, and the newer "no flow" film. The "high-flow" bond film is in a relatively uncured state before lamination, and tends to seep into the cavity during the lamination process, partially or completely filling the cavity, thus destroying the usefulness of the board. The "no-flow" type bond film will minimize seepage, but is much more expensive, and tends to trap air during the lamination process. Furthermore, due to the pre-cured nature of the material, it does not easily flow around the window edge. Thus the film may not form a good seal at the base of the cavity after lamination, even if the windows are painstakingly registered to high precision.
In order to ensure the integrity of the cavity during the lamination process, and at the same time maintain the quality of the board after lamination, techniques have been proposed which uses the high-flow bond film, such as in U.S. Pat. No. 4,680,075. Another document, U.S. Pat. No. 5,784,782, further describes a method of using a visco-plastic material which is laid over the registered windows to prevent the high flow bond film material from seeping into the cavity during lamination. These techniques, however, are still rather tedious, due to the window cutting and registering steps.